In the refining of lignocellulose-containing fibres by, for example, a disc refiner or a conical refiner at a low consistency of about 3 to 4%, the structure of the fibre wall is loosened, and fibrils or so-called fines are detached from the surface of the fibre. The formed fines and flexible fibres have an advantageous effect on the properties of most paper grades. In the refining of pulp fibres, however, the aim is to retain the length and strength of the fibres. In post-refining of mechanical pulp, the aim is partial fibrillation of the fibres by making the thick fibre wall thinner by refining, for detaching fibrils from the surface of the fibre.
Lignocellulose-containing fibres can also be totally disintegrated into smaller parts by detaching fibrils which act as components in the fibre walls, wherein the particles obtained become significantly smaller in size. The properties of so-called nanofibril cellulose thus obtained differ significantly from the properties of normal pulp. It is also possible to use nanofibril cellulose as an additive in papermaking and to increase the internal bond strength (interlaminar strength) and tensile strength of the paper product, as well as to increase the tightness of the paper. Nanofibril cellulose also differs from pulp in its appearance, because it is gel-like material in which the fibrils are present in a water dispersion. Because of the properties of nanofibril cellulose, it has become a desired raw material, and products containing it would have several uses in industry, for example as an additive in various compositions.
Nanofibril cellulose can be isolated as such directly from the fermentation process of some bacteria (including Acetobacter xylinus). However, in view of large-scale production of nanofibril cellulose, the most promising potential raw material is raw material derived from plants and containing cellulose fibres, particularly wood and fibrous pulp made from it. The production of nanofibril cellulose from pulp requires the decomposition of the fibres further to the scale of fibrils. In processing, a cellulose fibre suspension is run several times through a homogenization step that generates high shear forces on the material. This can be achieved by guiding the suspension under high pressure repeatedly through a narrow gap where it achieves a high speed. It is also possible to use refiner discs, between which the fibre suspension is introduced several times.
In practice, the production of nanofibril cellulose from cellulose fibres of the conventional size class can, at present, only be implemented by disc refiners of laboratory scale, which have been developed for the needs of food industry. This technique requires several refining runs in succession, for example 2 to 5 runs, to obtain the size class of nanocellulose. The method is also poorly scalable up to industrial scale.